Subject of the invention is a contact element for an electrical device comprised of a sliding key to be pressed down, with a flexible switching mat delimiting an area with electrical contacts at specific distances, and with an element protecting against electrostatic discharge designed to be connected to a ground terminal of the electrical device.
Such contact element is used especially often in keyboards for computers or consumer electronics equipment, and is known from practical applications. The protective element is usually designed as a metal plate located above the switching mat with a recess for the key. The user can generate electrostatic voltages of up to 25 kV, which are routed via the metal plate into the ground of the electrical device, preventing the electronic components of the electrical device from being damaged.
The disadvantage of the known contact element is that the metal plate is costly to manufacture and difficult to install.
The objective of this invention is to design a contact element of the aforementioned type, which can be manufactured in a highly economical manner.
The invention solves this problem with the specific characteristic that the protective element has an electrically conductive coating on the side of the mat facing the key.
The installation of the protective element on the switching mat automatically establishes protection against electrostatic discharge while the switching mat is being assembled. This makes a separate installation of the protective element unnecessary. Furthermore in comparison, depositing the coating during the manufacture of the switching mat can be done in a much more economical manner than the installation of the metal plate. This makes the contact element as introduced in this invention especially economical. Another advantage of the invention is that the protective element requires less installation space.
The manufacturing costs for the contact element described in this invention can be reduced further if the electrically conductive coating contains conductive lacquer or electrographic ink. Such conductive lacquer or electrographic ink is often used as contact within the switching mats, and contains particles with a specific electrical conductivity like, for example, graphite.
The electrically conductive coating could, for example, like the metal plate for the known protective element, have a very low impedance.
However, as a result, the user of the contact element described in this invention would receive a perceptible electric shock when the static electricity is being discharged. This is for the user perceptible shock can be avoided with an advanced design of the invention, namely, when the electrically conductive coating has a high impedance relative to metal.
Using another advanced design of the invention, the electrostatic discharge can be deflected especially reliably, if at least part of the key is coated with an electrically conductive coating.